如何创建一个对象，JNI？ [英] How to create an object with JNI?

问题描述

我需要使用NDK来实现一些功能集成到一个Android应用程序，从而JNI。

I need to implement some functions into an Android application using NDK and thus JNI.

下面是C code，我的担忧，我写道：

Here's the C code, with my concerns, that I wrote:

#include <jni.h>
#include <stdio.h>

jobject
Java_com_example_ndktest_NDKTest_ImageRef(JNIEnv* env, jobject obj, jint width, jint height, jbyteArray myArray)
{
    jint i;
    jobject object;
    jmethodID constructor;
    jobject cls;
    cls = (*env)->FindClass(env, "com/example/ndktest/NDKTest/Point");

//what should put as the second parameter? Is my try correct, according to what
//you can find in .java file? I used this documentation: http://download.oracle.com/javase/6/docs/technotes/guides/jni/spec/functions.html#wp16027

    constructor = (*env)->GetMethodID(env, cls, "<init>", "void(V)");
//http://download.oracle.com/javase/6/docs/technotes/guides/jni/spec/functions.html#wp16660
//Again, is the last parameter ok?

    object = (*env)->NewObject(env, cls, constructor, 5, 6);
//I want to assign "5" and "6" to point.x and point.y respectively.
    return object;
}    

我的问题都或多或少内code解释。也许也：是函数（jobject）确定

My problems are more or less explained inside the code. Maybe also: is the return type of the function (jobject) ok?

现在的NDKTest.java：

Now the NDKTest.java:

package com.example.ndktest;

import android.app.Activity;
import android.widget.TextView;
import android.os.Bundle;

public class NDKTest extends Activity {
    /** Called when the activity is first created. */
    public native Point ImageRef(int width, int height, byte[] myArray);
    public class Point
    {

        Point(int myx, int myy)
        {
            x = myx;
            y = myy;
        }

        int x;
        int y;
    }

    @Override
    public void onCreate(Bundle savedInstanceState)
    {

         super.onCreate(savedInstanceState);
         TextView tv = new TextView(this);
         byte[] anArray = new byte[3];
         for (byte i = 0; i < 3; i++)
             anArray[i] = i;
         Point point = ImageRef(2, 3, anArray);
         tv.setText(String.valueOf(point.x));
            setContentView(tv);     
    }



    static
    {
       System.loadLibrary("test");
    }
}

当我尝试运行code，这是行不通的。

When I try to run the code, it doesn't work.

推荐答案

由于点是一个内部类的方式来获得这将是

Since Point is an inner class, the way to get it would be

jclass cls = (*env)->FindClass(env, "com/example/ndktest/NDKTest$Point");

在 $ 约定内部类是不是真的清楚地记录在权威的规范，但盘踞在这么多的工作code，它是不太可能改变。尽管如此，它会的感觉的，如果你限制你的JNI code与顶级类的工作较为强劲。

The $ convention for inner classes is not really clearly documented in the authoritative specs, but is entrenched in so much working code that it's unlikely to change. Still, it would feel somewhat more robust if you restricted your JNI code to work with top-level classes.

您需要一个构造函数两个整数作为参数。该签名是（二）V ，这样：

You want a constructor that takes two ints as arguments. The signature for that is (II)V, so:

constructor = (*env)->GetMethodID(env, cls, "<init>", "(II)V");

接下来的时间，包括一些错误处理在code，如果你有其中的一部分不工作的线索！

Next time, include some error handling in your code, such that you'll have a clue which part of it doesn't work!

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